From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([208.118.235.92]:39304)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <rth7680@gmail.com>) id 1ULFRD-0003Ug-Fz
	for qemu-devel@nongnu.org; Thu, 28 Mar 2013 12:12:31 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <rth7680@gmail.com>) id 1ULFRA-0000Ng-KD
	for qemu-devel@nongnu.org; Thu, 28 Mar 2013 12:12:27 -0400
Received: from mail-ob0-x233.google.com ([2607:f8b0:4003:c01::233]:39225)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <rth7680@gmail.com>) id 1ULFRA-0000NZ-FJ
	for qemu-devel@nongnu.org; Thu, 28 Mar 2013 12:12:24 -0400
Received: by mail-ob0-f179.google.com with SMTP id un3so9357035obb.38
	for <qemu-devel@nongnu.org>; Thu, 28 Mar 2013 09:12:24 -0700 (PDT)
Sender: Richard Henderson <rth7680@gmail.com>
Message-ID: <51546BE3.2060509@twiddle.net>
Date: Thu, 28 Mar 2013 09:12:19 -0700
From: Richard Henderson <rth@twiddle.net>
MIME-Version: 1.0
References: <1364484781-15561-1-git-send-email-rth@twiddle.net>
	<1364484781-15561-8-git-send-email-rth@twiddle.net>
	<CAFEAcA8e-vR+dWdAaypOG-YFbTKHGTXyrBirYjzLTA0iv_5AhA@mail.gmail.com>
In-Reply-To: <CAFEAcA8e-vR+dWdAaypOG-YFbTKHGTXyrBirYjzLTA0iv_5AhA@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH v3 07/20] tcg-arm: Fold epilogue into
	INDEX_op_exit_tb
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-devel@nongnu.org, Aurelien Jarno <aurelien@aurel32.net>

On 03/28/2013 09:05 AM, Peter Maydell wrote:
> On 28 March 2013 15:32, Richard Henderson <rth@twiddle.net> wrote:
>> The epilogue on ARM is one pop instruction, that pops the return
>> address into PC.  Avoid the jump to jump for this case.  Use the
>> standard movi32 routine for loading the return value if it's easy.
> 
>> @@ -2025,8 +2023,7 @@ static void tcg_target_qemu_prologue(TCGContext *s)
>>      tcg_out_mov(s, TCG_TYPE_PTR, TCG_AREG0, tcg_target_call_iarg_regs[0]);
>>
>>      tcg_out_bx(s, COND_AL, tcg_target_call_iarg_regs[1]);
>> -    tb_ret_addr = s->code_ptr;
>>
>>      /* ldmia sp!, { r4 - r12, pc } */
>> -    tcg_out32(s, (COND_AL << 28) | 0x08bd9ff0);
>> +    tb_pop_ret = (COND_AL << 28) | 0x08bd9ff0;
>>  }
> 
> Why are we using a variable when it's always constant?

I just wanted the comments for the stmia and ldmia to be the same, and near one
another.  Would it be better if tcg_target_qemu_prologue was moved up in the
file, and tb_ret_addr made a static const (which the compiler should fold)?

E.g.

/* ldmia sp!, { r4 - r12, pc }  @ See the stmia definition below.  */
static const uint32_t tb_pop_ret = ...;

static void tcg_target_qemu_prologue(TCGContext *s)
{
   /* comment */
   /* stmia sp!, {r4-r12, lr} */
   tcg_out32(...);
   ...
}

> Also, please add a comment to the bottom of the qemu_prologue()
> saying something like
> 
> /* We never return here; we always return directly from generated
>  * code to our caller.
>  */

Sure.


r~